Welcome to Chapter 4 — DC Electrics: Batteries — Page 49, Lesson 52

Welcome to Chapter 4 — DC Electrics: Batteries. This is where we start looking at how an aircraft stores electrical energy when the main generation isn't available. Let me begin with the fundamental purpose of a battery in an aircraft. A battery provides an emergency source of power when the generator is not running, and it also provides power to start the engine. So think of it as your backup power supply and your engine-starting muscle all in one unit. Now, what is a battery made of? A battery is made up of a number of cells. Each cell converts chemical energy into electrical energy by a transfer of electrons from one material to another, which causes a potential difference between them. During that transfer of electrons, the chemical composition of the two materials changes. So the cell is literally consuming its own chemistry to produce electricity. We start with the simplest type: the primary cell. A primary cell consists of two electrodes immersed in a chemical called an electrolyte. The electrolyte encourages electron transfer between the electrodes until there is a potential difference between them. When the electron transfer ceases, the cell is fully charged, and the potential difference is approximately 1.5 volts between the two electrodes. Let me walk you through what happens when you connect that cell to an external circuit. When the positive and negative terminals are connected to an external circuit, electrons flow from the negative terminal to the positive terminal through the circuit. At the same time, more electrons are allowed to transfer inside the cell from the positive electrode to the negative electrode. So you have an external flow of electrons through your circuit, and an internal flow of electrons through the electrolyte. As this circulation of electrons continues, the negative electrode slowly dissolves in the electrolyte until it is eventually eaten away. At that point, the cell is "dead" and is discarded. This is a critical point: primary cells cannot be recharged. Once the chemical reaction has run its course, the cell is finished. That's why we call it a primary cell — it's a one-use device. Now, you'll also see the term "dry cell" used for primary cells, and Figure 4.2 illustrates that construction. But the key takeaway for you as a pilot is that primary cells are not what power your aircraft's main battery. We'll move on to secondary cells — rechargeable batteries — next, and that's where the lead-acid and nickel-cadmium batteries come in, which are the ones you'll actually find on aircraft.